The present invention relates to a digital data time division multiplex transmission technique, in which a plurality of digital data signals formed by different modulation schemes or different error correction codes are transmitted in different packets of a time division multiplex transmission, and at a receiver end, a desired digital data signal is selectively extracted among the transmitted plurality of digital data signals, and particularly relates to a digital data receiving apparatus, in which the transmitted plural digital data signals can be prevented from being interacted mutually upon an occasion that a transmission condition is degraded.
For instance, in a conventional television broadcasting, it is general that one kind of plural transmission schemes is selected. However, in BS digital broadcasting utilizing a broadcasting satellite as a relay station, it has been proposed to utilize commonly a rare relay station by a plurality of broadcasting enterprises or program suppliers. That is to say, it has been proposed to transmit a plurality of digital broadcasting programs via a single BS channel.
In this case, it is desirable that each of broadcasting enterprises can freely select a transmission scheme having a desired error tolerance against a low C/N (a ratio of received carrier wave to noise). This can be achieved in that when respective broadcasting enterprises transmit digital data signals (services) with desired transmission schemes, respective data signals are compressed on a time axis and are transmitted in a multiplex mode (time division multiplex). For instance, it is now assumed that a plurality of digital television signals produced by a plurality of broadcasting enterprises are transmitted in a multiplex mode in accordance with MPEG. Then, each of TS (Transport Stream) packets supplied from respective broadcasting enterprises has added thereto an identification flag of 13 bits, i.e. identifier called PID (Packet Identification) information for identifying broadcasting enterprises to which respective packets belong. By using such an identifier, at a receiver end, only a desired packet can be selectively extracted from the received packet stream, and desired picture and audio information can be decoded.
As stated above, the services supplied from respective broadcasting enterprises to respective users are transmitted in accordance with desired transmission schemes having desired error tolerances, i.e. desired error correction code and/or modulation scheme. For instance, TC8PSK modulation scheme, QPSK system, BPSK system and so on have been developed as the modulation scheme for the BS digital broadcasting, and further various error correction codes have been proposed. It is desirable that respective broadcasting enterprises can select any of these error correction codes and modulation schemes. For instance, it is desired that when there is not a fear that the transmission quality is degraded by a rail attenuation, the 8PSK system having a larger transmission capacity is used, but when the transmission quality might be degraded, the QPSK system or BPSK system having a higher tolerance against a rain attenuation is used.
In this case, depending upon a degree of degradation due to transmission conditions, certain data transmitted by a certain transmission scheme is correctly transmitted, but another data transmitted by another transmission scheme might not be used because a bit error rate is too high. In this case, it should be accepted that the data transmitted under a condition of a higher bit error rate could not be used, but since the PID information in the unusable packet is also subjected to the bit error, there is a certain possibility that this PID information might be accidentally changed into another PID information whose packet is transmitted correctly. If such an occasion arises, a service transmitted by the correctly transmitted packet might be disturbed (interdata interference). Therefore, it is desired that although a transmission packet of a certain broadcasting enterprise utilizing a certain modulation scheme or a certain error correction code is disturbed, this does not affect transmission packets of the other broadcasting enterprises.
The present invention has for its object to provide a digital data receiving apparatus, in which the above mentioned interference between data packets transmitted on the time division multiplex mode with different transmission schemes can be suppressed.
According to the invention, a digital data receiving apparatus for receiving a desired digital data signal among a plurality of digital data signals transmitted on the time division multiplex mode in accordance with different transmission schemes comprises:
a means for detecting whether or not a correct reception can be effected for each of said plurality of different transmission schemes; and
a means for replacing a transmission packet transmitted by a transmission scheme which is judged to be unable to perform a correct reception, by a signal which does not affect packets transmitted by another transmission schemes.
In a preferable embodiment of the digital data receiving apparatus according to the invention, said means for detecting whether or not a correct reception can be effected for each of said plurality of different transmission schemes includes a circuit for detecting a bit error rate of a signal which is transmitted by each of said different transmission schemes, and a comparator for comparing said bit error rate with a predetermined threshold value to judge that a correct reception can not be performed when said bit error rate exceeds said threshold value.
In another preferable embodiment of the digital data receiving apparatus according to the invention, said means for detecting whether or not a correct reception can be effected for each of said plurality of different transmission schemes includes a circuit for detecting an occasion that a bit error of a signal transmitted by each of said different transmission schemes could not be corrected.
Furthermore, in the digital data receiving apparatus according to the invention, it is preferable that said means for replacing a transmission packet transmitted by a transmission scheme which is judged to be unable to perform a correct reception, by a signal which does not affect packets transmitted by another transmission schemes comprises a circuit for replacing the relevant received packet into a null packet. Here, the null packet is a packet in which all bits except for first synchronizing bytes (47HEX) are 1.
In another preferable embodiment of the digital data receiving apparatus according to the invention, said detecting means is consisting of a bit error rate estimating means or an error correction impossible detecting means.